The invention relates generally to a capping device for an ink jet printer and more particularly to a device for securely and uniformly positioning a cap over an ink jetting orifice of an ink jet print head to seal the print head and then maintaining correct pressure and ink level within the print head.
A conventional ink jet printer typically includes a print head mounted on an electric machine which can be miniaturized. Ink is typically drawn to an appropriate level to ink jet nozzles by capillary action. When the print head is vibrated or tilted, such as when it is transported, ink typically flows backward from a front nozzle end of the print head to a level that is unsuitable for printing and can also spill out of the printer. In addition, ink at an ink jet nozzle can dry when the printer is not in use for a long period of time and interfere with printing. Both of these shortcomings of conventional printers adversely affect the ability of a printer to properly generate characters and images and undesirably increase printer down time.
To prevent ink from spilling from the printer or drying out, conventional printers have been fitted with capping devices. An example of a conventional capping device is described in Japanese Publication No. 15911/88 which describes a printer having a capping device designed to cover and seal the print head while the printer is not in use. The capping device includes a suction mechanism to draw the ink from an ink tank to a proper level in the print head so that the ink meniscus will be properly positioned at ink jet nozzles for printing.
This conventional capping device can often be effective in properly maintaining the meniscus level of ink when employed in conjunction with a print head in which ink in the ink tank is open to the air. If the tank is open to the air, the pressure in the tank is not reduced when ink is drawn from the reservoir and ink will not be siphoned back to the tank when the suction is released.
However, this capping device has been unsuitable for use in conjunction with an ink jet printer that includes a head damper and in an ink flow passage connecting an ink jet nozzle and an ink tank or another ink storage system in which the ink reservoir is not open to the atmosphere. As the suction device in the above conventional capping device draws the meniscus to a proper level at the ink jet nozzles, the pressure in the head damper becomes unacceptably low. Consequently, when the cap is removed to expose the ink jet nozzle to the atmosphere so that printing can occur, vacuum in the head damper siphons ink back into the head damper and lowers the meniscus to a level that is unacceptable for proper printing. Accordingly, this ink capping device does not adequately solve the problem of a lowered meniscus which can lead to imperfect ink discharge.
A conventional device for pressing a cap to a print head is described in Japanese Publication No. 15911/88 and is shown generally as capping device 110 in FIG. 11. Capping device 110 includes a cap support lever 53 pivotally mounted about a support lever fulcrum 53a. A first arm 53b of support lever 53 is pivotally mounted to a cap 52 at a cap fulcrum 52a. A second arm 53c of support lever 53 is rotatably coupled to a cam roller 56 in contact with a cam 55 having a caming surface 55'. Cap support lever 53 also includes a spring finger 53d coupled to a coiled tension spring 54. Tension from spring 54 constantly exerts a force to pivot cap support lever 53 clockwise and thereby urges cap 52 towards a closed sealed position against a print head 51. By selectively rotating cam 55, support member 53 can be selectively pivoted counterclockwise to displace cap 52 away from print head 51 to uncover an ink jet nozzles 51a to permit printing to occur.
Cap 52 is constructed and pivotally coupled to support lever 53 so that if print head 51 is unintentionally displaced longitudinally in the directions indicated by a double arrow A' with respect to cap 52, cap support lever 53 can pivot around fulcrum 53a in the directions indicated by a double arrow B' and cap 52 can pivot about fulcrum 52a in the directions indicated by double arrow C'. Accordingly, cap 52 will continue to be sealed against print head 51 during minor displacements of print head 51.
Cap 52 can only pivot in one direction with respect to print head 51. Thus, if print head 51 is displaced in a direction other than that of double arrow A', an improper non-uniform pressure distribution at a surface of cap 52 contacting print head 51 can occur. This can deform cap 52 and lead to an improper seal. The arrangement shown in FIG. 11 is only acceptable for certain types of ink jet printers. When cap 52 is sufficiently wide to cover a plurality of rows of nozzles included in a single print head, inadequate capping can occur more readily due to deformation of the cap from the uneven pressure distribution. An imperfect seal causes ink in the vicinity of the ink jet nozzles to dry which adversely affects ink discharge and can lead to ink leakage from the cap.
Another conventional ink jet printer capping device is described in Japanese Laid-Open Patent Application No. 260341/85. The capping device includes a cap having a thin tube disposed therethrough and an intermediate portion of the thin tube includes an expansible diaphragm-carrying chamber.
Still another conventional capping device is described in Japanese Patent Laid-Open No. 273855/87 which describes a device similar to an ink capping device shown as 101 in cross-section in FIG. 10. Capping device 101 includes a protective cap 42 for covering ink nozzles 41a of a print head 41. Before printing occurs, cap 42 is removed from the surface of print head 41 by a cap opening and closing device which is not shown in FIG. 10. A pair of tubes 47 and 49 are operatively coupled to cap 42 and are in fluid communication with cap interior 42a of cap 42 and with ink jet nozzles 41a. Tube 47 is coupled to and is in fluid communication with an expansible chamber 45 which includes a flexible diaphragm 45a. Expansible chamber 45 is operatively coupled to and is in fluid communication with another tube 48 which is coupled to a valve 46 for regulating the pressure within chamber 45 and thereby, within cap interior 42a. Tube 49 is coupled to the inlet of a suction pump 44 for reducing the pressure within cap interior 42a. Flexible tubes 47, 48 and 49 are formed of materials which are highly resistant to the corrosive effects of conventional inks.
When the meniscus of ink in print head 41 falls below an acceptable level, suction pump 44 applies suction to tube 49 and thereby to the ink passageways of print head 41 through nozzles 41a to draw the meniscus in print head 41 back to a suitable level. A valve 46 is provided to relieve unacceptable pressure levels that can develop within chamber 45.
Expandable chamber 45 is included in capping device 101 to absorb environmental pressure changes. Accordingly, atmospheric pressure changes will not generally adversely affect the volume of air in communication with interior 42a so that ordinary atmospheric pressure changes will not unacceptably displace the meniscus of ink within print head 41.
The ink located within print head 41, nozzles 41a and flexible tubes 47, 48 and 49 contains water. When the ink jet printer is exposed to high temperatures for an extended period of time, water in the ink will evaporate into water vapor and the volume and partial pressure of the water vapor in tubes 47, 48 and 49 will increase. Initially, expansible chamber 45 will expand and absorb this volume increase. However, as the partial pressure of water vapor increases the partial pressure of air molecules within capping device 101 decreases and becomes less than the partial pressure of the outside atmosphere. Tubes 47, 48 and 49 of a conventional capping device are typically formed of materials such as polyethylene or polytetrafluoroethylene or other materials which have a high resistance to the corrosive effects of ink, but allow air molecules to pass through relatively easily. As the partial pressure of air molecules within the tubes decreases, air will pass through the walls of tubes 47, 48 and 49 and cause the volume of gas therein to increase.
Eventually, the volume increase of gas cannot be absorbed by expansible chamber 45 and the internal pressure within capping device 101 will unavoidably begin to increase. At an ambient temperature of 40.degree. C., the internal pressure can increase up to about 55.3 mmHg, the saturated vapor pressure at 40.degree. C. This internal pressure within capping device 101 will overcome forces supporting the meniscus of ink at the front end portion of ink jet nozzles 41a and cause the meniscus to displace backwards to an unacceptable level. This leads to imperfect ink discharge and increases printer down time.
Accordingly, it is desirable to provide a capping device for an ink jet printer which will overcome these shortcomings of the prior art capping devices.